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- If we let a speaker
make one short burst,

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we'll create a pulse wave.

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We can find the speed of sound

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by looking at the speed
of this compressed region

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as it travels through the medium.

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In non-humid air at 20 degrees Celsius,

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the speed of sound is about

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343 meters per second
or 767 miles per hour.

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We can also watch the speed of sound

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of a repeating simple harmonic wave.

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The speed of the wave
can again be determined

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by the speed of the compressed regions

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as they travel through the medium.

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Note that the speed of sound does not mean

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the speed of the air molecules
as they move back and forth.

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The air molecules are
moving with the speed,

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but by the speed of sound,

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we mean the speed of the disturbance

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as it moves through the air molecules.

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We call sound a longitudinal wave

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because the wave is traveling parallel

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to the line traced out by the
oscillations of the medium.

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The other type of wave
is a transverse wave.

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Transverse waves happen
when the wave velocity

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points perpendicular to the
oscillations of the medium.

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Waves on a string or waves
on the surface of water

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are examples of transverse waves.

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If we look at a graph
of the air displacement

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versus position of the air,

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we can see that as the wave travels

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the shape of this wave
travels to the right.

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So, the speed of a sound wave can be found

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by finding the speed of the peaks

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or the speed of the valleys

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or the speed of any single
point on the wave shape.

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To figure out a formula for
the velocity of a sound wave,

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let's look closely at
what's happening here.

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Watch one of the air molecules.

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It takes one period for this molecule

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to move back and forth
through a full cycle.

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During this time, the wave
shape has moved forward

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one complete wavelength.

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This is because the wave has to overlap

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with its initial shape after one period,

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because the molecule has
to be back where it started

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after one period.

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Now, since speed is defined
to be the distance per time,

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the speed of a sound wave has to be

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the wavelength of the wave

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divided by the period of the wave.

51
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Since the wave is traveling forwards

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one wavelength per period,

53
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or since the frequency is defined
to be one over the period,

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we can rewrite this formula as

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speed equals wavelength times frequency.

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This formula is accurate
for all kinds of waves,

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not just sound waves,

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because a wave has to move one
wavelength for every period.

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Be careful.

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When looking at this equation,

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you might think that if you adjust

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the setting on your speaker
and increase the frequency

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you'd also be increasing
the speed of the sound wave,

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but that's not what happens.

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If you increase the frequency,

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the wavelength will decrease
by that same factor,

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and the speed of the sound
wave will remain the same.

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In fact, there's nothing
you can do to the speaker

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that would increase the speed of sound.

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So, how can we change the speed of sound?

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Well, the only way to
change the speed of sound

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is to change the medium or
the properties of the medium

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that the sound wave is traveling in.

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So, to change the speed of sound in air,

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you can change things like
the temperature of the air

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or the humidity of the air
or the density of the air,

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or you can swap out the air
entirely for another material,

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like water or helium or a metal.

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All of these changes to the medium

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would affect the speed of sound.

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People often think that
changing the amplitude

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will change the speed of a
sound wave, but it won't.

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If we create a sound pulse
with a large amplitude,

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it won't travel any
faster than a sound pulse

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with a small amplitude in the same medium.

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It will just be louder.

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In other words,

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yelling won't cause
anyone to hear you faster,

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they'll just hear a louder sound

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when the sound wave
arrives at their location.

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So remember, the speed of a sound wave

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is determined entirely by
the properties of the medium

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through which it's traveling.